Tube bending mandrel



Dec. 24, 1968 J. cs. SCHMITT TUBE BENDING MANDREL Filed June 30, 1966 INVENTOR JOHN G. SCHM/TT AGENT ATTORNEY United States Patent O 3,417,602 TUBE BENDING MANDREL JohnG. Schmitt, 2516 Norcross St., Hillcrest Heights, Md. 20031 Filed June 30, 1966, Ser. No. 562,452 12 Claims. (Cl. 72466) ABSTRACT OF THE DISCLOSURE This invention is directed to a waveguide mandrel which is capable of forming a waveguide surface free of scratches or burrs. Further, the corners are at right angles and the waveguide body is free of any sidewall wrinkles. A plurality of crescent shaped pins each having a concave face that matches an adjacent circular pin are placed onto a flexible elongated rectangular strap. The flexible strap is then provided with a pulling head that pulls the mandrel through the waveguide.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

The present invention relates to apparatus for use in bending tubes, and more particularly to a mandrel for use in effecting small radius seamless bends in rectangular wave guide tubes.

It is well known that wave guides used for transmitting high frequency electromagnetic energy must possess accurate internal dimensions which are uniformly formed and must be provided with an internal finish which possesses no interior scratches or burrs which are detrimental to electrical results. Wave guides are bent in a spiral having a small radius at the beginning thereof in order to provide a relatively long wave guide over a short space between operative components of a system. Heretofore, wave guide bending methods have left untold interior scratches and burrs which are detrimental to electrical results. Other drawbacks have been that the dimensional sizes cannot be repeated and accurately held; also, inside corners of the wave guide have not always been precisely at good right angles. Further, drawbacks reside in the fact that previously =bent wave guides have sidewall wrinkles. Each of the mentioned drawbacks has an effect on transmission of electromagnetic energy in the previously made wave guides. It is therefore necessary in effecting bends in wave guides to uniformly support the interior thereof with a flexible mandrel in order to preserve as far as possible the original interior crosssectional dimensions of the tubes and to size the inner dimensions thereof, wher they may be slightly off, while preventing scratching of the interior surface of the wave guide by the supporting mandrel in order to eliminate arcing of the high frequency energy or other unwanted changes in the electrical transmission characteristics of the wave guide.

It is therefore an object of the present invention to provide an articulated mandrel for supporting the interior of a rectangular wave guide tube during the bending thereof.

Another object is to provide a mandrel which will substantially eliminate scratching of the interior surface of a wave guide tube during the bending thereof.

Still another object is to provide a mandrel by which internal uniform dimensions may be maintained for successively made wave guides while forming internal finishes which "are free of interior scratches and burrs which are detrimental to electrical results.

Other objects and advantages of the present invention will become apparent from an examination of the following detailed description when taken in conjunction with the accompanying drawing, wherein:

FIG. 1 is a side view illustrating the relative parts;

FIG. 2 is a cross-sectional view illustrating in more detail the relative parts of the mounting head of the mandrel;

FIG. 3 represents a sizing segment; and

FIG. 4 represents one of a plurality of crescent-shaped segments used in forming the mandrel of the present invention.

The wave guide mandrel of the present invention comprises an elongated flexible body such as a spring steel strap upon which a plurality of crescent-shaped pins and at least one sizing pin are mounted. One end of the flexible body is provided with a pressure block for holding the pins on the flexible body and the opposite end of the flexible body is secured within a mounting head and a cover plate. The mounting head of the mandrel is secured to a hydraulic pull cylinder for withdrawal from a wave guide and for loading the mandrel by reversing the hydraulic pull cylinder.

Now referring to the drawings there is shown by illustration, a preferred embodiment of the present invention. As shown, the wave guide mandrel comprises an elongated flexible body 10 of spring steel which may be made of a single or of two or more thin strips assembled side by side. The spring steel strap 10 is provided with suitable small apertures at one end for securing thereto a pressure block 11 by suitable rivets. The pressure block is suitably formed with all edges radiused, polished and then chrome plated to provide a high finish, surface hardened and scratch free surface. The opposite end or pull end of the strap is provided with larger apertures for the purpose of securing the strap to a rigid mounting head 12 by which the mandrel is secured to a hydraulic pull cylinder (not shown) for withdrawal from a wave guide and for loading the mandrel by reversing the hydraulic pull cylinder.

The mounting head 12 is formed by a main body portion 15 which is also secured to the hydraulic pull cylinder and by a cover plate 16. The main body portion 15 is formed of an elongated rectangular piece of steel which has a linear cavity 14 milled therein to a depth greater than the center line thereof. An axial slot 17 having a width and length slightly greater than the thickness and width of the steel strap of the pull end of the steel strap is milled in one end of the main body of the mounting head for passage of the pull end of the steel strap into the cavity 14. The bottom surface of the cavity within the main body portion is provided with three threaded holes 21 and two blind holes 22. of larger diameter. A cover 16 for the cavity is provided with three countersunk holes 23 that are in alignment with the threaded holes 21 in the mounting head and two larger holes 24 that are in alignment with the blind holes 22 in the main body of the mounting head. A carboloy pin 25 is silver soldered in each of the larger holes 24 in the cover plate and are of a length such that they extend into the blind holes in the mounting head when the cover is in place. Theend corners of the cavity, the main body portion, and the cover plate are rounded; and all of the surfaces of each are ground to remove any burrs or scratches thereon and then chrome plated to provide a smooth surface.

The pull end of the spring steel strap extends into the main body of the mounting head through the slot in the end thereof and the apertures within the pull end of the steel strap are spaced such that they are in alignment with the apertures within the cover plate and the main body of the mounting head. The cover plate is placed into posi tion over the pull end of the spring steel strap within the cavity within the main body portion such that the carboloy pins extend through the larger holes in the spring steel strap and into the blind holes in the bottom surface of the main portion of the mounting head. Screws 26 are passed through the countersunk holes within the cover plate through the aligned holes in the spring steel strap and threaded into the threaded holes in the bottom of the main body of the mounting head. The screw heads fit within the countersunk portion of the holes within the cover plate such that the heads are even with the outer surface of the cover plate. Also, the outer surface of the cover plate when in position within the cavity over the pull end of the steel strap is even with the outer surface of the main body of the mounting head to provide a smooth surface.

Mounted on the length of the spring steel strap between the mounting head and the pressure block are a plurality of crescent-shaped pins or segments 31 which are assembled with the round surface of one pin adjacent the concave surface of its adjacent pin, as shown. The end segment'32 adjacent to the pressure block is of cylindrical shape and of slightly greater diameter and length than the crescent-shaped segments for sizing the inner dimension of the wave guide. Each of the cresent-shaped segments may be made of drill rod material, slotted, at 33, along the axis with the distance from the outer end of each segment to the end of the slot being of equal dimen sions. The concave face 34 is made along the length of the segment such that a plane parallel with the sides of the slot in the segment passes through the axis of the segment and the axis of the circle that forms the concave face on the segment, with the axis of the circle parallel with the axis of the segment. The top and bottom surface of the end segment 32 and each of the segments 31 are made with a fiat portion 35 across the center thereof with the surface 36 sloping away from the flat portion having an angle relative thereto. The fiat portion extends across the end surface perpendicular to the plane through the axis parallel to the sides of the slot such that the flat surface is perpendicular to the sides of the slot through the pin. All of the edges of the ends of each pin, the slot, and the edges of the concave surface are radiused, polished and then the surfaces are chrome plated to provide a smooth surface.

In assembly of the device, the spring steel strap is formed into the proper width and thickness as desired. The spring steel strap may be made of a single piece or of separate thin straps secured together. The apertures are made in the pull end and also the opposite end, to which the pressure block is then secured. Each of the separate crescent shaped pins or segments are formed, ground, polished and chrome plated and the mounting head and cover plate are formed and prepared for assembly. A spring steel strap of a desired length is used and the sizing pin is placed over the strap and placed adjacent the pressure block then a plurality of crescent-shaped pins or segments are successively placed onto the strap subsequent to the sizing pin. The crescent-shaped pins are secured such that the concave surface is adjacent to the cylindrical surface of the adjacent pin. Suflicient crescent-shaped pins are placed on the strap to fill the entire length of the strap up to the point to which the pull end of the strap is secured to the mounting head. The pull end of the spring steel strap is inserted into the slot in the end of th mounting head and secured therein with the apertures therein aligned with the apertures within the mounting head. The cover plate with the carboloy pins is then placed into position with the carboloy pins inserted into the proper apertures within the spring steel strap and forced into the blind holes or apertures in the main body portion of the mounting head. Subsequent thereto the screws are inserted into the countersunk holes of the cover plate and screw-threaded into the threaded holes within the bottom of the milled out cavity portion of the mounting head. The Wave guide mandrel is now secured together and is ready to be fastened by apertures 37 to a hydraulic pull cylinder for insertion into a wave guide and withdrawal therefrom.

The mandrel can be loaded into the wave guide front or backwards which ever is convenient for the operator. For illustrative purposes only and for left or right hand bending, a mandrel made in accordance with the present invention and used for a radius of 1% inch minimum or 20 inch maximum contours may be made'with the following dimensions. The spring steel strap is 27 inches long and ft of an inch wide which comprises two pieces of spring steel 0.018 inch thick with floating side strips on each side thereof of 0.010 spring steel. The floating side strips are used as protection of the spring steel center strips to prevent the slotted edges of the segments, from biting into the pull strips. The floating side strips are only as long as the length upon which the segments are secured and are not secured in the mandrel head. Thus, the floating side strips are free to shift during bending of the mandrel. The apertures in the pull end of the spring steel strap'are made with the first aperture inch from the end with each of the successive apertures 1% inches apart on centers, wherein the first, third, and fifth aperture is'of sufficient size to receive therethrough a 9/ screw whereas the second and fourth hole has a diameter of inch. The pressure block secured to the end of the spring steel strap is of 1 inch width and /2 inch in length with the outer end rounded. The sizing pin may be made in various sizes of outside diameter and in length in steps of 0.001 inch starting at 0.250 inch in diameter and 1.118 inch in length. The crescent-shaped pins are made with an outside diameter of 0.249 inch and a length of 1.118 inch with the radius of the concave face thereon of 0.254 inch, and with a slot therethrough having the dimensions of {3& by %1 inch. The slot through the sizing pin and the crescent-shaped pins are of the same dimensions. The radius of the end corners of the segments is 0.020 inch. For a spring steel strap 27 inches in length, 110 pieces of the crescent-shaped pins are required and whereas only one sizing pin is required. The mounting head has a length of 12% inches, a width of 1.118 inches and a thickness of 0.245 inch. The cavity in the main body portion of the mounting head starts of an inch from the end toward the crescent-shaped pins and has a length thereof of 5 inches, a width of of an inch and with a depth of 0.143 inch. The slot through which the pull end of the spring steel strap passes has a width of 7 inch by ,41 inch, and the face of the mounting head at the slotted end has a radius of 0.254 inch. The first drilled and tapped hole in the bottom of the cavity is /2 inch from the beginning of the cavity toward the slotted end of the main body portion and the apertures therein 1% inch on centers from the first hole with the first, third and fifth holes being drilled and tapped for 9 screws and whereas the second and fourth holes have a /8 inch diameter hole having a depth of 0.016 inch. The cover plate has a /s inch thickness by inch width and 6 inches long which has a tight fit in the cavity of the main body portion of the mounting head. The first, third, and fifth apertures in the cover plate are countersunk to receive therein a size 10 screw wheras the second and third apertures are of a inch diameter to receive therein a /8 inch by 0.187 length carboloy pin which is silversolde'red therein. The last 3 /2 inch length of the mounting head furtherest away from the pull end of the spring steel strap is provided with suitable apertures therein for mounting the wave guide mandrel to a suitable hydraulic pull cylinder. It is obvious to one skilled in the art that the diameters of the crescent-shaped pins, the sizing pins and the number of pins may be changed according to the desired size of the wave guide which is to be coiled.

It can be seen that a mandrel made in accordance with the construction as set forth above simplifies changing the separate segments for any length needed. Removal of three flat head screws from the cover plate permits one to replace the length of spring steel pull bar that is needed and which is reloaded with crescents as required to fill the length of spring steel pull bar with the desired size crescents for bending a desired waveguide.

The mandrel mounting head has been described as being made with carboloy pins silver-soldered to the cover plate. It will be obvious to others that the cavity in the main body portion of the mounting head may be milled such that the pins are a part of the main body extending outwardly from the bottom of the cavity and of sufficient length to fit into blind holes in the cover plate. Thus, the cover plate will have two less holes through the surface thereof to provide a smoother finished surface.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. A tube bending mandrel, which comprises:

an elongated rectangular flexible body,

a plurality of separate elongated axially slotted pins assembled onto said flexible body, means secured to one end of said flexible body for preventing removal of said separate pins from said one end of said flexible body, and a mandrel head means,

said mandrel head means being secured to the opposite end of said flexible body from that of said means secured for preventing removal of said pins for pulling said flexible body and said separate pins assembled thereon through a tube to be shaped.

2. A tube bending mandrel as claimed in claim 1, wherein;

said plurality of pins include at least one cylndrical pin and a plurality of crescent-shaped pins.

3. A tube bending mandrel as claimed in claim 2, wherein;

said cylindrical shaped pin is slightly larger in diameter and length than that of said crescent shaped pins.

4. A tube bending mandrel as claimed in claim 3, wherein;

the curvature of the concave surface of said crescentshaped pins is the same as the curvature of the cylindrical portion thereof.

5. A tube bending mandrel as claimed in claim 4, wherein;

said plurality of pins are assembled onto said flexible body with adjacent faces having the same radius of curvature. 6. A tube bending mandrel as claimed in claim 3, wherein;

each of said plurality of pins have a flat surface across the mid-section of each end with the remainder of the end surface sloping away from the flat midsection. 7. A tube bending mandrel as claimed in claim 1, wherein;

said pin is generally of cylindrical shape, a concave face along the length of said pin, and a slot through said pin along the axis thereof, with the slot bisecting said concave face. 8. A tube bending mandrel as claimed in claim 7, wherein;

said pin surfaces are ground, milled and polished. 9. A tube bending mandrel as claimed in claim 8, wherein;

said pin is chrome plated. '10. A tube bending mandrel as claimed in claim 9, wherein;

the end surfaces of said pin have a flat section across the middle thereof with the top surface sloping away from the flat section to the outer edges of the pin. 11. A tube bending mandrel as claimed in claim 1; wherein,

said flexible body comprises more than one member. 12. A tube bending mandrel as claimed in claim 11; wherein,

said flexible body includes at least one main flexible member with a floating strip along opposite sides thereof.

References Cited CHARLES W, LAN'I-IAM, Primary Examiner.

B. J. MUSTAIKIS, Assistant Examiner. 5 

